The present invention relates to an improved electrical resistance welding gun and, more particularly, to an improved transformer for an electrical resistance welding gun.
Electrical resistance welding is, of course, well-known and electrical resistance welding guns are frequently used in the fabrication of vehicles to weld together parts of the vehicle such as floor pans, fenders, roofs, hoods, doors, frames, etc. Electrical resistance welding guns typically comprise first and second electrodes moveable relative to each other and oppositely disposed relative to one another for welding, with each electrode being attached to a body member (called an arm), and an electrical transformer including primary windings and secondary windings. One type of electrical resistance welding gun provides the transformer at a remote location from the gun itself and the electrical power is coupled to the electrodes by means of an elongated cable. This type of welding gun allows the electrodes to be taken to the workpiece while the transformer is relatively stationary at a location remote from the workpiece. In such a welding system, the majority of the electrical power necessary for the system is lost transferring power from the transformer to the electrodes. That is, the power required to weld the workpiece is normally quite small as compared to the total power requirement of the welding system. Thus in a welding system of the type just described the actual weld (i.e., I.sup.2 R between the electrode tips) consumes less than 2 kilowatts, the remainder of the welding gun (excluding the cable) may consume 15-25 kw and the cable itself is a primary source of energy loss such that a ten foot cable may consume as much as 200 kw.
A second type of welding gun utilizes the transformer as a structural part of the arm of the welder. Such a welding gun is disclosed in my U.S. Pat. No. 4,233,488 issued Nov. 11, 1980. Since the transformer is physically adjacent the electrode, a long cable is not necessary. This avoids the problem of power loss due to a long cable from the transformer to the electrode. My aforementioned co-pending application describes an improved transformer which may be utilized as a structural part of the body member or arm of an electrical resistance welding gun.
A third type of electrical resistance welding gun uses a transformer attached to the arm or body member of the welding gun, rather than as a structural part of the welding gun arm. The transformer is adjacent the electrodes and a long cable is not necessary. Since transformers may burn out, it is beneficial to have a transformer attached to the arm of the welding gun because such transformers may be easily replaced. Also, if the power requirements of the system change, a different capacity transformer may be readily attached to the arm of the welding gun.
While a basic objective in an electrical resistance welding gun is to conserve energy by the reduction of line current, there are various conflicting sub-problems which occur. Specifically, the minimum line current necessary for welding is related to the load current needed for welding in proportion to the transformer "turns ratio", which is the ratio of the number of primary turns to the number of secondary turns. To minimize line current, a higher "turns ratio" is needed. However, in order to provide a sufficient secondary voltage to overcome the total impedance of the system, a lower "turns ratio" is needed. But, once a lower "turns ratio" is provided, the result is the need for a higher line current.
Thus, the desire to reduce line current has been frustrated by the necessity of a sufficiently high line current in conjunction with a sufficiently low "turns ratio" to provide not only the necessary power for welding but also the necessary secondary voltage to overcome the impedance of the welding system.
A problem which arises with transformers for welding guns is the extreme amount of heat generated by the gun. The heat may have a pronounced deleterious effect on the transformer and thus systems have been developed to dissipate the heat and to cool the transformer. In the use of prior art electrical resistance welders it has been customary to provide a cooling member through which a coolant is flowed for the purpose of cooling the transformer of the welding gun. The cooling member must be thermally conductive, to draw the heat from the transformer, and a cooling fluid or coolant is flowed through the cooling member. Typically, copper tubes are used as the cooling member because of the high thermal conductivity of the copper. However, the use of a cooling member increases the weight of a welding gun and the use of a metal cooling member increases not only the weight but also the resistance and the reactance of the welding gun.
Portable electrical resistance welding guns which are moved to a workpiece are often attached to "robots", i.e., programmable machines which move the welding gun to a desired position, cause the electrodes to close upon the workpiece, and control the application of the welding current through the electrodes to weld the workpiece.
If the electrical resistance welding gun is to be utilized with a robot, the use of a remote transformer and a long cable from the transformer to the electrodes still results in the energy loss problem described above. Thus, the present day approach to the use of electrical resistance welding guns in conjunction with robots dictates that the transformer will either be a structural part of the arm of the welding gun or attached to the arm of the welding gun. In either event, however, the use of a robot results in an additional problem, namely, the criticality of the weight of the transformer. Because of the combination of the speed at which robot-controlled welding guns are operated, especilly in the fabrication of vehicles, it is necessary to reduce the weight of the welding gun as much as possible. For example 3600 welds per hour is a desirable speed for a robot-controlled welding gun. At such a rate, which is one weld per second, the cycle of the robot-controlled welding gun would be: one-quarter second to move into welding position and grab the workpiece between the electrodes; one-quarter second to weld; one-quarter second to hold the welded workpiece while the weld cools; and one-quarter second to release the workpiece and move out of welding position. Since one-fourth of each cycle is the actual welding, such a system has a 25% duty cycle. With the prior art welding guns it was not possible to make a transformer of sufficiently light weight to be moved by the robot at the aformentioned rate while still providing cooling which would prevent the transformer from burning up at the 25% duty cycle. On the other hand, if the transformer (including the cooling member) was of a sufficient size to provide the necessary cooling at a 25% duty cycle, the transformer would be too heavy to be moved by robots operating at the desired speed of 3600 welds per hour. Thus, prior to the present invention, industrial robot-controlled welding guns used the technique of a large, remote transformer and a cable for transferring the welding power to the electrodes. This, of course, resulted in relatively inefficient, high energy loss systems.
The present invention overcomes the aforementioned problems relating to electrical resistance welders in general and in particular to electrical resistance welders for use in conjunction with a robot, by providing an improved transformer for an electrical resistance welding gun.